SSDs are not reliable after a (relatively-low compared to spinning
disk) number of writes.
They may significantly boost performance if used on the "journal"
storage, but will suffer short lifetimes for highly-random write
patterns.

I agree with this statement in general, however, my understanding is that Cassandra NEVER does random writes. It only ever does large sequential writes. Cassandra could potentially be the perfect use case for MLC (multi-level-cell) SSD's.

around 1800 col/sec per node, 3kb columns, reading is the same.Data will be deleted after 4 hours.

Hmm, only keeping the data for 4 hours could present some unique challenges with Cassandra since it does not actually delete the data (it only tombstones the data). There are several factors that play into when exactly the data actually goes away